Inspection of electron phosphor screens



NOW 1955 FlTZ-HUGH B. MARSHALL 2,725,483

INSPECTION OF ELECTRON FHOSPHOR SCREENS Filed May 20, 1950 3 .a l3 2 m '4 i 3 {iii 8 g Wire Grid Lens g 9 Aluminum X-Rcy Tube Lead Gloss Fluorescent Material WITNESSES: INVENTOR flg F Fitz-Hugh B. Marshall.

United States PatenttO INSPECTION or ELECTRON PHOSPHOR SCREENS Application May 20, 1950, Serial No. 163,246

9 Claims. (ill. 250-53) My invention relates to inspection apparatus, and in particular relates to arrangements for determining the uniformity in texture, resolving power, and homogeneity of the electron phosphor layers of fluorescent screens used for reproducing, in cathode ray tubes, television receiving tubes, and various image intensifying tubes, an intensified picture image. It produces outstanding results in the inspection of relatively thin phosphor layers.

Arrangements have been devised, such as those described and claimed in the United States Patent 2,555,545 of Lloyd P. Hunter and Richard L. Longini, issued on or about June 5, 1951, entitled Image Intensifier and assigned to the assignee of the present application which employs an electron lens system cooperating with fluorescent screens to reproduce a desired optical image at a greatly intensified level of brightness. My invention is particularly useful in testing these screens.

One of the most important components of such an arrangement, is a fluorescent screen comprising a layer of some electron phosphor, such as zinc sulphide, on which electrons corresponding in space distribution to the optical image being intensified are incident.

In order that the light image produced on this fluorescent screen shall correspond in intensity, point by point with the electron image, it is necessary that the layer of phosphor be as uniform and perfect in resolution as possible.

My present invention relates to an arrangement for inspecting such screens during the course of manufacturing to reassure the required uniformity. It is particularly adapted to use in quantity production of such screens.

One object of my invention, accordingly, is to provide a method of determining the uniformity and homogeneity of thin fluorescent screens.

Another object of'my invention is to provide an arrangement for inspecting any electron phosphor screen, especially when backed by a reflective coating, for imperfections, variations of thickness or sensitivity, and the like.

Other objects of my invention will become apparent upon reading the followingdescription taken in connection with the drawing in which the single figure is a diagrammatic illustration of an inspection apparatus embodying the principles of my invention.

Turning in detail to the drawing, the fluorescent screen comprises an annular member 1 having a periphery suiting it to be sealedglass or otherwise attached to form a wall portion of an electrical discharge device.

.Within the annular member 1 is supported ascreen 2 comprising a thin glass plate 3 coated on its interior surface with a substantially uniform layer 4 of a. fluorescent material such as zinc sulphide. The glass plate may be sealed at its outer edge to the annular memher 1. The face of the fluorescent layer may be coated with a thin layer of aluminum or nickel, thin enough to be penetrated by electrons passing through an electron lens which is positioned inside the electrical dis charge tube. Methods of constructing such a screen are ICC described and claimed in the United States Patent 2,586,304 of J. W. Coltman et al., issued February 19, 1952, and entitled Protection of Phosphors From Attack by Alkali Vapors, which is likewise assigned to the assignee of this application.

For inspection purposes, the annular member 1 may be affixed to any suitable support member (not shown) in the path of X-rays emanating from an X-ray tube 6 of any suitable type. For present purposes, the tube 6 preferably has a line focal spot 7 which is arranged in a manner well known in the art, so that the X-rays to be used are emitted at a glancing angle from the target and tend to form the sharpest detail shadowgraphsf practical for a given distance of the source from the target. Tubes known in the art as X-ray diffraction sources of X-rays are of this type and I have found them satisfactory. More to the point, for a given high resolution, the focal-spot-to-screen distance can be small so as to permit high intensity at the screen, which effect is further aided by the line focus principle. The rays from line focus 7 pass through a window 8 which is preferably of beryllium. For the present purpose and for most purposes, it is highly desirable to employ relatively soft, long wavelength X-rays (i. e., from 1 to 5 Angstroms) from X-ray tubes having beryllium windows or other thin or low atomic number windows, especially adapted to efliciently pass such X-rays which are well known in the art. Where, as in the following description, straight wires or rods are used to cast shadows on the screen during testing, the line focus is positioned parallel to the wire.

' After passing from the focal spot 7 from the window 8 the X-rays impinge on the fluorescent screen 2. If only resolution testing is of primary importance, as in the present image tube case, resolution is most important. But in the general case of testing overall uniformity it is desirable that the -Xrays induce luminosity which will be substantially uniform if the thickness and quality of the phosphor forming the screen is uniform. On the other hand, irregularities in thickness and texture of the fluorescent material will result in corresponding non-uniformity in the luminosity of the various portions of the screen. For the latter more general purpose it is desirable that the focal spot and window arrangements in the X-ray tube 6 shall produce an extremely high intensity, broad and uniform beam of X-rays. For the more special case of resolution testing, the emphasis is on high intensity and sharpness of shadow detail.

Light from the surface of the screen 2 then passes through a heavy plate of lead glass 11 on the opposite side of which it may be viewed by the eye 12 of the inspector. An optical magnifying lens 13 may be usefully employed to scan the area of the screen 2 to aid in'de'termining its texture and resolving power. The lead glass for operator protection may be quickly removed when the X-rays are off, for changing screens in assembly line production testing.

The ability of the screen 2 to properly resolve fine detail images projected upon it may be determined by positioning a fine wire grid 14 close to the inner surface of the screen 2. In this case the line focal spot is preferably' used, and the fine wires of this grid are made parallel to the line focal spot 7.

- Instead. of inspecting the screen 2 by eye as above described, a permanent record of the inspection may be made by photographing the screen. The plate 11 of lead glassfwill protect the eye of the observer, and'will also protect the camera used for photography, from X-rays which pass unabsorbed through the screen 2. Since electron phosphors and the glass upon which they are deposited are, in general, often poor absorbers of X-rays when in thin layers, such provisions for preventing harm to the inspector using the equipment are necessary. Suitable screening by lead plates may likewise be provided in other parts of the apparatus for protection purposes.

The X-ray diffraction type of source has the valuable property of producing a line focus which is a fine, narrow line capable of projecting a high intensity beam which is highly collimated in its transverse dimension, and can be brought much closer to the wires of the testing-grid and the phosphor screen than ordinary X-ray sources. It further produces readily a soft X-ray beam, and I take fullest advantage of the intensity of the soft X-rays by employing a beryllium or other similar window to sulfuse the phosphor with rays of a wavelength which it can effectively absorb even though it is relatively thin. The line focus of the diffraction tube enormously in.- creases the intensity of the light from the phosphor rela-v tive to that obtainable with circular focal spots of equal focusing power; the correspondence between the direction of the line focus and absorbing wires parallel to it accounts for the possibility of thus increasing the radia tion intensity without losing focusing effect.

While the use of my X-ray device is particularly effective for phosphor screens having a metal or other opaque backing, the device has advantages over ultraviolet and/or electron radiation of phosphor layers devoid of such backing; for example, a. more uniform irradiation throughout the thickness of the phosphor layer.

To give a specific example of an inspection carried out with the above-described arrangement, a fluorescent: screen having a layer of Zinc sulphide approximately .001 centimeter thick and coated with a layer of Al metal about 0.00002 centimeter thick was positioned about ten centimeters away from the focal spot of a line-focus beryllium-window X-ray difiraction tube of the above-described type operated at 45 kilovolts and milliamperes input.

A 30 line per millimeter grid of tungsten wire was placed 40 mils from the electron phosphor layer. Under such conditions the light given off by the electron phos-v phor was found sufiicient to overcome the normal illumination present in a laboratory and to permit easily detailed inspection of the tungsten grid pattern in a slightly darkened room. The above-mentioned screen had a diameter of one inch. Though the target-toascreen dis! tance was centimeters, measurements showed that a distance of as little as 3 centimeters (with tenfold brightness increase) would have still permitted the desired reso! lution.

While I have described my apparatus as employed for inspection of zinc sulphide electron phosphor screens, it may likewise be applied for inspecting fluorescent screens in television receiver tubes, cathode ray tubes and other similar electron optical screens. In fact, it is with these that the most general use of the invention is expected.

I cl im as. my in n i n:

1 An apparatus for inspecting a screen of electron phosphor about 0.001 centimeter thick having an opaque backing which comprises an X-ray tube having a beryllium window, means for supporting said screen in the path of X-rays projected through said window and means having a high absorbing power for X-rays positioned in the path of a portion only of the X-rays passing from said window to said screen.

2. Means for inspecting a zinc sulphide type electron phosphor screen about 0.001 centimeters thick which comprises an X-ray tube having a line focal spot and a beryllium window, means forsupporting said screen in the path of X-rays passing through said window from said fo al spo and a m r al. h i g a inear.- dg Parallel to the line of said focal spot having a high absorbing power for X-rays in the path of said X-rays which have passed from said focal spot through said window.

3. An apparatus for inspecting a thin electron phosphor screen having an opaque backing, said apparatus comprising an X-ray dififraction source of X-rays producing a line focus, means for supporting said screen in the path of said X-rays, and a filament of material which is a poor transmitter of X-rays positioned parallel to said focus line in said path ahead of said screen.

4. Apparatus for inspecting a layer of zinc sulphide about 0.001 centimeter thick having an opaque backing which comprises means for supporting said layer, means for producing a beam of soft X-rays and projecting said beam through said layer.

5. Apparatus for inspecting a layer of zinc sulphide about 0.001 centimeter thick and covered with a layer of metal which comprises means for supporting said layer, a line focus X-ray tube having a beryllium window positioned to project said X-rays through said layer, and wires of material opaque to said projected X-rays positioned parallel to said line focus in the path of said projected X-rays.

6. Apparatus for inspecting a layer of zinc sulphide electron phosphor about 0.001 centimeter thick which comprises means for supporting said layer, means for producing a beam of X-rays of 1 to 5 Angstroms wavelength, and projecting said beam onto said phosphor.

7. Apparatus for inspecting a layer of electron phosphor which comprises means for supporting said layer, a linear type source of X-rays, means for projecting said X-rays onto said phosphor, and means relatively opaque to X-rays and having a linear edge substantially parallel to said source in the path of said X-rays ahead of said phosphor.

8. Apparatus for inspecting a layer of electron phosphor about 0.001 centimeters thick which comprises means for supporting said layer, an X-ray tube having a beryllium window, means for projecting said X-rays through said window onto said phosphor, and means relatively opaque to X-rays in the path of said X-rays ahead of said phosphor.

9. Apparatus for inspecting a thin layer of zinc sulphide type phosphor which comprises means for supporting said layer, a linear source of X-rays, means for projecting said X-rays onto said phosphor, and means having a rectilinear edge whichis relatively opaque to X-rays and is substantially parallel to said source in the path of said X-rays ahead of said phosphor.

References Cited in the file of this, patent UNITED STATES PATENTS 587,883 Thomson Aug. 10, 1897 1,370,640 Granger Mar. 8, 1921 1,590,971 Goetze June 29, 1926 1,942,007 Slack Jan. 2, 19 34 2,055,188 Wappler et al. Sept. 22, 1936 2,057,325 Bouwers et al Oct. 13, 1936 2,120,916 Bitner June 14, 1938 2,140,269 Pelkus et al Dec. 13, 1938 2,189,623 Bourland Feb. 6, 1940 2,297,478 Kallmann Sept. 29, 1942' 2,310,567 Atlee et al. Feb. 9, 1943 2,430,969 Young Nov. 18, 1947 2,433,129 Land Dec. 23, 1947 2,475,596 Dawson July 12, 1949 2,549,987 Parrish et al. Apr. 24, 1951 2,557,662 Kirkpatrick June 19, 1951 

1. AN APPARATUS FOR INSPECTING A SCREEN OF ELECTRON PHOSPHOR ABOUT 0.001 CENTIMETER THICK HAVING A BERYLBACKING WHICH COMPRISING AN X-RAY TUBE HAVING A BERYLLIUM WINDOW, MEANS FOR SUPPORTING SAID SCREEM IN THE PATH OF X-RAYS PROJECTED THROUGH SAID WINDOW AND MEANS HAVING A HIGH ABSORBING POWER FOR X-RAYS POSITIONED IN THE PATH OF A PORTION ONLY OD THE X-RAYS PASSING FROM SAID WINDOW TO SAID SCREEN. 